Optical spectroscopy entails passing optical radiation through a sample, often referred to as an analyte, and inferring properties of the analyte from measurements performed on the optical radiation. For example, trace gas detection can be spectroscopically performed by performing measurements to detect the presence or absence of spectral absorption lines corresponding to the gas species of interest. Optical spectroscopy has been intensively developed over a period of many decades, and various ideas have been developed to improve performance.
One such idea can be referred to as cavity-enhanced spectroscopy, in which the analyte is disposed within an optical cavity (i.e., an optical resonator). The cavity can enhance the interaction between the analyte and the optical radiation, thereby improving spectroscopic performance. For example, in cavity ring-down spectroscopy (CRDS), a form of cavity enhanced absorption spectroscopy, the absorption is measured by way of its effect on the energy decay time of an optical cavity. Increased absorption decreases the decay time, and vice versa. As another example, cavity enhanced absorption spectroscopy (CEAS) can also be employed to increase the sensitivity of absorption spectroscopy in connection with direct absorption measurements.
A significant alignment issue faced in many implementations of cavity-enhanced spectroscopy is selectively exciting the lowest order transverse mode of a passive optical cavity with an external optical source while minimizing excitation of the higher order transverse modes of the cavity. The theoretical condition for providing such selective mode excitation is well known in the art, and is often referred to as “mode matching”. For example, suppose radiation in the lowest order transverse mode of an optical cavity would be emitted from the cavity as a Gaussian beam having certain parameters (e.g., waist size w0, waist position z0) along a beam axis L. In this example, radiation provided to the cavity as a Gaussian beam with waist size w0 and waist position z0 along beam axis L is mode matched to the lowest order transverse mode of the resonator, and will selectively excite the lowest order transverse mode of the cavity.
In experimental practice, mode matching is often optimized by monitoring the excitation of the higher order transverse modes, and by adjusting the system to minimize such excitation. For example, the spatial mode pattern can be monitored, or a fast detector can be employed to monitor transverse mode beating. In an article by Lee et al. entitled “Optimization of the mode matching in pulsed cavity ringdown spectroscopy by monitoring non-degenerate transverse mode beating” (Appl. Phys. B 74 435-440 (2002)), mode matching is optimized by introducing an intentional misalignment of a degenerate cavity. Such misalignment breaks the mode degeneracy, and results in transverse mode beating at relatively low frequencies, which do not require a fast detector to measure. Mode matching to the cavity is optimized by minimizing the amplitude of the slow mode beating, and then the misalignment is removed to complete alignment.
Despite the use of such methods for optimizing mode matching, it remains difficult and/or time consuming to optimize mode matching in practice. Accordingly, it would be an advance in the art to provide improved ease of mode matching to an optical cavity.